Low-Dimensional Polymer

January 21, 2025 – Researchers from Technische Universität Dresden, Ulm University, the Max Planck Institute of Microstructure Physics, the University of Göttingen (all Germany), the University of Cambridge (UK) and the University of Hong Kong have synthesized a new electrically conductive coordination polymer, Cu₅BHT, with a low-symmetry structure. It conducts like a metal and carries current about eight times better along one crystal direction than along the others. Aberration-corrected electron microscopy at Ulm University resolved the unusual arrangement of its building blocks.

Electrically conductive coordination polymers (ECCPs) and two-dimensional conjugated metal–organic frameworks (c-MOFs) combine metal ions and organic molecules into extended networks. Their electrons are shared within each layer through π–d conjugation, the overlap of the organic π orbitals with the metal d orbitals, and between neighbouring layers through electronic coupling. Depending on the composition, this makes them semiconductors or even metals.^1,2 Because they are assembled bottom-up from molecules, their electronic structure can be tailored at the molecular level, which makes them candidates for next-generation electronic and quantum applications.³

The ECCP family has grown rapidly, above all with periodic networks of square, honeycomb and Kagome lattices. These are usually built from square-planar MX₄ linkages, where M is a metal ion and X is O, NH or S, combined with conjugated organic ligands such as benzenehexathiol (BHT),^1,3 hexahydroxybenzene (HHB)⁴ and hexaiminotriphenylene (HITP).⁵ Nonporous ECCPs built from benzene-derived units, such as Cu_xBHT (x = 3, 4, 5.5),^1,3 Cu₃HHB⁴ and Cu₄DHTTB (DHTTB = 2,5-dihydroxy-1,3,4,6-tetrathiolbenzene),⁶ are particularly interesting: they adopt a Kagome lattice with a high density of ligands and metal ions, which favours extended π-electron conjugation and strong π–d interactions. The result is well-dispersed electronic bands and narrow or even zero band gaps.⁴

Over the past decade, conductivity and charge carrier mobility of ECCPs have improved markedly, but their anisotropic charge transport, the dependence of conduction on the direction within the crystal, is still being explored.^4,7,8,9,10 A low-symmetry ECCP with in-plane electrical anisotropy had not been realized experimentally. The team set out to make one.

Synthesis and Structural Characterization of Cu₅BHT
The researchers grew Cu₅BHT on a water surface in a Langmuir trough, in four steps from the copper solution to the transfer of the film (Fig. 1a, 1b). The result was a black film of about 50 cm², visible to the naked eye. The box describes the recipe.

Transmission electron microscopy (TEM) showed that the film consists of densely packed crystal domains, interspersed with smaller crystallites and an amorphous thin film (Fig. 2). The main domains have an asymmetric hexagonal shape with lateral dimensions from 200 nm to 1 µm (Fig. 1c). Elemental mapping by energy-dispersive X-ray spectroscopy in the scanning TEM (STEM-EDX) confirmed a uniform distribution of carbon, sulfur and copper across the entire domain (Fig. 1d).

Crystallographic Analysis and Phase Purity
A low-magnification TEM image shows an asymmetric hexagonal Cu₅BHT flake (Fig. 3a). Selected-area electron diffraction (SAED) recorded perpendicular to the crystal surface gave sharp spots, indicating a single crystal, arranged in a hexagon whose angles are not all 120° (Fig. 3b). These unequal angles point to a low-symmetry lattice. The first-order spots lie at 1.35 nm⁻¹, corresponding to a lattice spacing of 7.4 Å.

To reach higher spatial resolution while keeping radiation damage low, the team used low-dose aberration-corrected high-resolution TEM (AC-HRTEM) at 80 kV with a dose rate of 3.2 × 10³ e⁻ Å⁻² s⁻¹. The image in Fig. 3c shows well-aligned Kagome-like lattices without noticeable defects or domain boundaries, in excellent agreement with the crystal structure of Cu₅BHT. The measured lattice distance of 7.4 Å matches the SAED value.

In the magnified image, individual atomic positions are clearly resolved. They reveal an asymmetric structure made of two kinds of secondary building units (SBUs): square-planar CuS₄ and non-planar Cu₂S₄, which differ in the contrast of their copper atoms (Fig. 3d, 3e). According to the study, the Cu₂S₄ units form layers along the (110) and (1-10) crystal directions, connected by chains of alternating benzene rings and CuS₄ units along the (010) direction.

Fast Fourier transforms (FFT) of the AC-HRTEM images showed the same distorted six-fold pattern as the SAED (Fig. 4). This identified the longest diagonal of the hexagonal crystals as the (010) direction; it lies between the two angles of less than 120°.

X-ray scattering on a macroscopic film confirmed the structure (Figs. 3f, 3g); the box gives the details. Cu₅BHT crystallizes in the orthorhombic P2gg space group with the unit cell parameters a = 14.69 Å, b = 8.48 Å and c = 3.45 Å. Simulated AC-HRTEM images, SAED patterns and projection profiles agreed excellently with the experiment and confirmed the phase purity of Cu₅BHT (Fig. 3e). From these results, the team built an atomic model for density functional theory (DFT) calculations, which shows a distinct local structure of low symmetry (Fig. 3h).

Chemical Stability and Charge Transfer Mechanism
In Cu₅BHT, copper is present as Cu⁺, although the synthesis starts from Cu²⁺. The reduction is likely driven by electron transfer from sulfur. The crystals also proved chemically and thermally robust. The box below explains both; Figs. 5 and 6 are inside the box.

Electronic Properties and In-Plane Anisotropic Conductivity
According to the theoretical calculations, Cu₅BHT is a metal, and its structure itself predicts a preferred direction: well-conducting chains of CuS₄ units run through slabs of less-conducting Cu₂S₄ units, all oriented along one crystal direction.

To test this on single crystals, the team connected crystals to Cr/Au electrodes with platinum wires written by a helium ion beam (Fig. 7a). Compared with conventional gallium ion beams, helium ions offer higher resolution and damage the sample less. Linear four-point-probe (4pp) current–voltage curves showed Ohmic contacts (Fig. 7b), consistently at all six electrode–polymer interfaces.

Between 300 and 30 K, the conductance increased monotonically on cooling along all three crystal directions, (010), (110) and (1-10) (Fig. 7c), a hallmark of metallic behaviour. At room temperature, the conductance along (010) exceeded that along (110) and (1-10) by an anisotropic factor, A = σ₍₀₁₀₎/σ₍₁₁₀₎ or σ₍₀₁₀₎/σ_(1-10), of about 8 (Fig. 7c, inset), and the factor grew further on cooling. The order σ₍₀₁₀₎ > σ₍₁₁₀₎ ≈ σ_(1-10) agrees with the theoretical predictions and is the first experimental demonstration of in-plane electrical anisotropy in a low-symmetry ECCP. For the other Cu_xBHT phases (x = 3, 4, 5.5), no comparable anisotropy has been observed at the device level.

The study shows that the same ingredients, copper and BHT, can form a new, low-symmetry network whose structure fixes the direction of current flow. “This work demonstrates a novel low-symmetry ECCP and highlights its potential for achieving in-plane electrical anisotropy,” the authors conclude. Direction-dependent conductors are of interest for electronic components that exploit anisotropy, as in inorganic 2D materials such as ReS₂, where anisotropic transistors have already been combined into digital inverters.¹⁰ The atomic structure behind the effect was resolved by low-dose aberration-corrected HRTEM at Ulm University.

Resource:
Wang, Z., Un, H.-I., Liu, T.-J., Liang, B., Polozij, M., Hambsch, M., Pöhls, J. F., Weitz, R. T., Mannsfeld, S. C. B., Kaiser, U., Heine, T., Sirringhaus, H., Feng, X. & Dong, R. (2025).
A low-symmetry copper benzenehexathiol coordination polymer with in-plane electrical anisotropy.
Angew. Chem. Int. Ed., 64(13), e202423341.
https://doi.org/10.1002/anie.202423341

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